In known high bit rate wireless transmission systems, the signals transmitted by the transmitter reach the receiver along a plurality of different paths. When they are combined at the receiver, the phase differences between the various rays having travelled paths of different lengths give rise to an interference pattern likely to cause fading or significant degradation of the signal.
Furthermore, the location of the fading changes over time depending on changes in the environment such as the presence of new objects or the passage of people. This fading due to the multiple paths may lead to significant degradation, in terms of both the quality of the signal received and system performance.
To combat fading, the technique most often used is a technique with spatial diversity. This technique consists inter alia in using a pair of antennas with large spatial coverage such as two antennas of the patch type combined with a switch. The two antennas are separated by a length which must be greater than or equal to λ0/2 where λ0 is the wavelength corresponding to the operating frequency of the antenna. With this type of antenna, it is possible to demonstrate that the probability of having both antennas simultaneously in a region of fading is very low. Furthermore, by virtue of the switch, it is possible to select the branch connected to the antenna having the highest signal level by examining the signal received via a control circuit.
The above solution has the main drawback of being relatively bulky. Consequently, the applicant has proposed various alternative solutions to the solution described above. These solutions are applicable to antennas of the slot type supplied by a line/slot transition and which make it possible to obtain radiation diversity.
Research has therefore been carried out on an antenna of the slot type, such as an annular slot supplied by a tangential line/slot transition. An antenna of this type is shown in FIG. 1. This antenna is produced on a substrate 1 such as the Chukoh Flo CGP500 substrate where Er=2.6, TanD=0.0018 and the height h=0.76 mm. It comprises an annular slot 2, the perimeter of which is of the order of k′λs where k′ is an integer and λs the wavelength guided in the slot.
As shown in FIG. 1, this annular slot 2 is supplied by a line/slot transition which is substantially tangential at the point P. The line/slot transition consists of a microstrip line 3 made on the substrate 1, this microstrip line being at a distance y from the point of tangency to the slot 2. The length of the microstrip line 3 between its end 3′ and the point P is about kλm/4, where k is an odd integer and λm the wavelength guided in the microstrip line. Furthermore, the characteristic impedance of the microstrip line is chosen so as to provide 50 ohms at the port 1. In this case, the coupling between the slot and the microstrip line is of electromagnetic type. To have maximum coupling between the exciting microstrip line and the slot, it is necessary to be placed in a short-circuit plane for the microstrip line. Thus, the coupling is optimized by adjusting the distance y between the slot 2 and the exciting line 3. Since the coupling takes place over a certain region on either side of the short-circuit plane for the microstrip line, broadband behaviour is obtained for the antenna excited in this way, as given in Table 1 below:
TABLE 1Y (mm)−0.25 0+0.25+0.5Matched bandwidth (%)14.314129.5
FIG. 2 also shows the reflection coefficient S11 of an annular slot 2 as a function of the frequency for the various values of y given in Table 1. These curves give the matching of the annular slot to the said values. In this research, it is simply mentioned that two annular slots excited symmetrically by a tangential supply line radiate in phase opposition. This therefore results in radiation in the zero axis.
However, contrary to this assertion, the applicant has noticed that, in a structure of the above type with positioning of the microstrip line with respect to the slots so that one is in a short-circuit plane of the microstrip line, the two annular slots radiate in phase, which gives constructive radiation along the axis having linear polarization of very high purity.